The present invention relates to control of permeability in subterranean oil-bearing formations, and more specifically relates to plugging excessively permeable water channeling zones in waterflood operations.
In the production of oil from subterranean formations, it is usually possible to recover only a small fraction of the total oil present in the formation by so-called primary recovery methods which utilize only the natural forces present in the reservoir. To recover oil beyond that produced by primary methods, a variety of supplemental production techniques have been employed. In these supplemental techniques, commonly referred to as secondary or enhanced oil recovery operations, a fluid is introduced into the oil-bearing formation in order to displace oil to a production zone including one or more production wells where the oil is brought to the surface. The drive fluids used in such operations include liquids such as water and various hydrocarbons, and gases such as hydrocarbon gases, carbon dioxide, etc. Often the most cost effective and desirable secondary recovery methods involve the injection of an aqueous or carbon dioxide flooding medium into an oil-bearing formation, where a number of injection and offset production wells have been arranged in a given pattern to produce the field.
While conventional waterflooding is generally the most cost effective method for obtaining additional oil from a reservoir, it has a number of shortcomings. Foremost among these shortcomings is excess water and decreased oil production in some of the offset producing wells in the field and not in others, which results in increased production costs and reduced oil production rate. The uneven production pattern usually appears after waterflooding has been on-going for some time and is thought to result from the tendency of injected flood water to eventually find a low resistance flow path around or through a partially depleted oil-bearing zone. This prevents uniform water injection into all oil-bearing zones evenly, and the resulting uneven water production in wells in a given waterflood field. In extreme cases, the waterflood channeling continues until a water breakthrough occurs such that large quantities of water drive fluid may channel directly from the injection well to a production well. Further, in this event of uneven distribution, significant quantities of oil may be bypassed and left unproduced in low permeability zones unless measures are taken to plug the high permeability bypass or so called xe2x80x9cthiefxe2x80x9d zones.
To solve the problem of undesired channelization in formations, voluminous previous work in the field has sought to chemically form precipitates within the subterranean formations which are capable of sealing off the highly permeable zones or channels so that the water flood drive fluid would be diverted to the under-swept low permeability oil containing regions of the reservoir. The process for controlling permeability of subterranean formations is usually referred to as xe2x80x9cprofile control.xe2x80x9d
In previous experiences, oil/water emulsions, gels formed by crosslinking polymers, etc., have been used for forming channel blocking precipitates which are relatively rigid. These channel blocking techniques, in which two or more separate fluids may be injected, have been applied with varying degrees of success. These channel blocking agents have been used in different types of reservoirs, and under diverse reservoir conditions of pressure, temperature, acidity, etc.
Accordingly, it is an object of this invention to seal off water producing zones of high permeability in a waterflood operation, without affecting less permeable oil producing zones.
It is a more specific object to seal off highly permeable subterranean zones which have been cooled by flood water that is much colder than the reservoir fluids.
A still more specific object is to seal off highly permeable subterranean zones near the well bore penetrating the zone.
Yet another object of this invention is to increase the efficiency of a drive fluid passing through a formation and thereby increase the yield of hydrocarbon fluids.
According to the present invention the foregoing and other objects and advantages are attained in a method for profile control in waterflood recovery operations carried out in a subterranean formation having both moderately permeable and less permeable zones. In a waterflood operations it is desired to enhance production from both zones, however, the oil flow from a more permeable formation is generally depleted before depletion of the less permeable zone. Continued water flood operations result in xe2x80x9cwater breakthroughxe2x80x9d which occurs either through or bypassing the moderately permeable zone. When water breakthrough occurs flood water will be diverted from the low permeability oil producing zones into the channels of a highly permeable zone where it will replace the original subterranean fluid, and produce large quantities of water through the production well. At that time the waterflood operation is ceased and a hydrate forming hydrocarbon gas is injected into the waterflood breakthrough zone. The injected gas on contact with the flood water will form a solid gas hydrate in the highly permeable channels of the breakthrough zone and reduce its permeability. Accordingly, subsequent water drive fluid will be diverted to less permeable oil-containing zones to improve production rate of the oil field.
In a preferred embodiment of this invention the water breakthrough zone will become cooled by the flood water to a temperature that is much colder than the original subterranean fluid. Then the hydrate forming gas is injected through a water injection well to contact the water in the highly permeable zone and form a solid hydrate near the perforations of the injection well to seal off the highly permeable zone taking the water. In an alternate embodiment, the hydrate forming gas is injected through the production well to form a zone of solid hydrate near the perforations of the production well to block water flow into the production well.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description and the drawings, wherein there is illustrated and described only one of several preferred embodiments of the invention. As will be realized several details of this invention are capable of modification in various obvious aspects without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative, and not as restrictive in nature.